Silverlight, Microsoft's platform for interactive Web applications, will be supported for Linux clients by a planned browser plug-in. Developed by Novell, the plug-in will help Microsoft's Silverlight platform match the platform-independence of Adobe's Flash, which in turn could help Silverlight compete for mindshare with Web developers. The move suggests that Microsoft considers Flash a serious enough threat to abandon its traditional reluctance to develop any software for the Linux platform.

Silverlight is based on the Windows Presentation Foundation (WPF) and is Microsoft's platform for Rich Internet Applications (RIAs)—Web applications that have features and functionality traditionally associated with desktop applications but run locally in a Web browser through a browser plug-in. Like Flash, Silverlight is cross-platform, and Microsoft has already developed a version of the browser plug-in for the Macintosh. However, Microsoft had not announced any support for Linux. The planned plug-in, known as Moonlight, will be developed and supported by the Mono team at Novell, a group of developers who previously independently developed a Linux version of the .NET Common Language Runtime.

In the past, Microsoft has encouraged third parties to produce Linux or Unix versions of technologies such as COM, but did little to assist them or otherwise guarantee their success. The agreement with Novell is different because Microsoft is providing Novell with important technical information. Specifically, Novell will get access to the test suites and detailed technical specifications used by Microsoft to develop the Windows and Mac versions of Silverlight. These specifications are far more detailed than what is publicly available about Silverlight for developers on Microsoft's Web site.

In addition, Microsoft will provide the video codecs needed to allow Moonlight to play video encoded in Microsoft's VC-1 format. No source code will be provided for the codecs and they will not be bundled with Moonlight—customers will need to go to Microsoft's Web site to download them, and they will be licensed for use only in a Web browser.

Novell plans to develop Moonlight versions of Silverlight 1.0, which is programmable only via JavaScript, and Silverlight 1.1, which is based on the .NET Framework, and will distribute them for the major Linux distributions. Initial plans call for supporting the Firefox browser on Linux—there are no plans to support other Unix-based OSs, such as Solaris, or other browsers, such as Opera.

Currently in their infancy, RIAs pose the same long-term threat to Microsoft that Java did: if they become widely used in place of desktop applications, they could make alternate OSs, such as the Mac and Linux, more practical for consumers and business users and dampen demand for OS upgrades. Although Microsoft would probably prefer that RIAs stay a niche, it would much rather control the dominant RIA platform than cede it to a competitor. In addition, the move presents very little risk to Microsoft: Linux is much more successful in corporations on the server than it is on the desktop, and producing Silverlight for Linux is unlikely to make any businesses switch from Windows to Linux.

The home page for the Moonlight project is www.mono-project.com/Moonlight.

Silverlight and RIAs are explained in "Silverlight Details Emerge, Expression Studio Ships" on page 33 of the June 2007 Update.